Study: Mammograms Lead to Many False-Positive Results

In 2009, a government advisory group sent women and their doctors into a frenzy when it rolled back recommendations for annual breast cancer screening for most women. Saying that routine mammograms could potentially do more harm than good, the panel advised women to delay regular screening until age 50, instead of age 40, and even then, to get tested every other year instead of annually.

That recommendation ran counter to that of many professional and advocacy groups, but it appears justified — at least according to the latest study to quantify the benefits and harms of frequent mammography screening. The study involving 170,000 women between the ages of 40 and 59 found that over 10 years, 61% of women getting annual screens were likely to receive false positive results, compared with 42% of women getting screened every two years. Among those getting annual mammograms, 7% to 9% would go on to get biopsies that turned out not to be cancerous; that rate was 5% to 6% in women having biennial mammograms.

A test that turns out not to be cancer may sound like good news, except that false-positive mammography results can be physically and psychologically harmful. Women can develop complications from infections due to biopsy, or they may have to undergo invasive procedures to remove lesions that might not have been malignant in the first place. And the anxiety of worrying about a potential cancer diagnosis can also be damaging to women’s health.

“These results add additional support that people should be screened every other year and not every year,” says the study’s lead author Karla Kerlikowske, director of the women’s veterans comprehensive health center at the San Francisco Veterans Affairs Medical Center. “There is no added benefit to doing mammograms every year. And we’re increasing harms without any added benefit. So if the benefit is staying constant, and the harm is increasing, then it doesn’t make sense to recommend annual screening.”

The study found that annual screening was not conclusively more effective than biennial screening at picking up dangerous late-stage cancers.

So why have women been urged to get tested every year once they turned 40? The reasons for that, argue Kerlikowske, may be more social than scientific. Unlike with heart disease, for example, there aren’t many things women can do to prevent breast cancer before it happens; the best way to stave off cancer is to catch and treat tumors as early as possible, by finding them through mammograms. So screening became the stand-in for good preventive care.

What’s more, strong advocacy groups that successfully raised awareness and much-needed research dollars for breast cancer care may have also pushed yearly screening as way for women to take their health into their own hands and proactively fight one of the leading cancer killers among women.

But, increasingly, studies are showing that more may not be better. The current study by Kerlikowske and her colleagues is also the first to follow the same women over a period of time and evaluate their mammogram results and subsequent treatments. Previous analyses of the benefits of mammography have looked at different populations of women at one point in time, and compared yearly or less frequent screening to positive or negative results. By tracking the same women over a decade, the current study was able to get a more accurate measure of how many false-positives mammograms generate.

If women follow the government’s current guidelines, which call for biennial screening between age 50 and 74, they should need only 12 mammograms during their lifetime, fewer than half of what they would receive if they follow previous advice.

That same government group, the U.S. Preventive Services Task Force (USPSTF), also released on Wednesday recommendations for cervical cancer screening, advising Pap tests once every three years in women ages 21 to 65 (the USPSTF’s previous recommendation in 2003 suggested biennial Pap tests for women in their 20s). Pap tests involve taking a scraping of cells from the cervix and analyzing it for signs of cancer.

The panel is also discouraging the use of increasingly popular tests for human papillomavirus (HPV), the virus that causes most cervical cancer. The USPSTF advises against HPV testing in women under 30, and finds that there is insufficient evidence at this time to gauge the balance of benefits and harms of HPV testing — alone or in combination with Pap tests — in women older than 30.

Again the question is one of false-positives. HPV testing is sensitive enough to pick up signs of the cancer-causing virus, but is it specific enough to identify cancer versus non-cancer? Despite advice by the American Cancer Society and other groups that say HPV testing can be an option for women older than 30, the government panel concludes that there isn’t enough data to make a recommendation for or against the use of these tests.

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.